Many electronic systems are designed for continuous operation, so that they are always able to respond to input signals. In order to save energy, these systems often have a rest mode or sleep mode, power only being supplied to those circuits that are essential for detecting input signals and which allow the idle system circuits to be switched on quickly. Input signals are normally detected by an electronic switching circuit supplying current to these idle circuits. For instance, recently introduced keyless locking systems for automobiles are able to lock or unlock automobiles in response to a radio-frequency signal transmitted by a transmitter of an electronic key. The locking circuit includes a self-polling ultra-high frequency (UHF) receiver circuit and an anti-interference, low-noise amplifier (LNA). Since the locking system should be able to respond to UHF signals, the power consumption of the circuits is kept to a minimum so as to prevent the car battery from being drained. The self-polling UHF receiver is able to be switched between a sleep mode and a run mode. The LNA may be switched in tandem so that it is supplied with power only when the UHF receiver is energized.
Conventional systems have achieved this by using a voltage comparator to monitor the output voltage of the UHF receiver circuit. When the UHF receiver switches from sleep state to operating state, the voltage comparator detects the change in the output voltage and supplies current to the LNA. According to this method, the output voltage of the UHF receiver is not well regulated. As a result, the LNA may mistakenly be energized due to electric noise and temperature fluctuations. Moreover, the sensitivity of the system is reduced if the LNA is not switched on in tandem with the UHF receiver.